Motor vehicles are commonly provided with brake systems to retard the rotation of the vehicle wheels. While all electric (non-hydraulic) brake systems have been proposed, most vehicles in use today employ a hydraulic or an electro-hydraulic braking system. In a conventional hydraulic vehicle braking system, the brake pedal is operatively connected to a master cylinder. The movement of the brake pedal causes a piston within the master cylinder to move, thereby forcing hydraulic fluid throughout the brake system and into cylinders located at each wheel. The pressurized hydraulic fluid then causes a piston located within the wheel brake cylinders to move. The movement of the brake piston causes a first friction surface to move into contact with a second friction surface operatively connected to the rotating wheel, thereby braking the wheel. A typical electro-hydraulic brake system includes a similar hydraulic system but additionally includes one or more sensors, such as a force sensor coupled to a vehicle's brake pedal, which develops a signal which is indicative of a driver's demand for braking. This signal is sent to an electronic control unit which in turn operates an electric motor to drive a pump to send the pressurized hydraulic fluid to the vehicle's brakes to develop the demanded braking force. Such electro-hydraulic brake systems are typically able to electronically control the brake pressure at each of the vehicle's wheels independently of the brake pressure at other ones of the vehicle's wheels.
The ability to independently control the braking force at each of the vehicle's wheels, together with certain special sensors, enables operation of a vehicle brake system in various special modes of operation. One of these special modes of operation is an anti-lock braking mode of operation, commonly referred to as ABS (for Anti-lock Brake System). Sensors in the vehicle brake system monitor the speed of the vehicle's wheels during braking. If the braking force demanded at a brake for a vehicle wheel causes the wheel to slip, the brake system can momentarily reduce the braking force of the brake at that wheel to allow the wheel to stop slipping, and thus provide optimal braking for the vehicle.
Another of these special modes of operation is traction control. During vehicle acceleration, a vehicle wheel may lose traction, and begin to spin. In the traction control mode of braking, the brake system is electronically actuated, without the driver stepping on the vehicle brake pedal, to individually brake the spinning wheel. When the wheel has slowed sufficiently to regain traction, the brake is released.
In most hydraulic and electro-hydraulic braking systems, solenoid valves are used to control the brake pressure in the brake lines. Solenoid valves may be digitally controlled in that the solenoid is either energized or deenergized and the valve is thereby moved to either a fully open position or a full open position. Partially open or throttled positions of the valve are brief transient positions during movement between the fully closed and the fully open position. During use, digitally controlled solenoid valves respond rapidly to actuation signals, which causes a fluid hammer effect. This problem is shown in FIGS. 1 and 1a. FIG. 1 shows a graph of a pressure profile during the operation of a conventional ABS in a typical light vehicle brake system. The horizontal axis represents time, with each division corresponding to 100 milliseconds. Both wheel speed and brake pressure are indicated on the vertical axis. Graph line A indicates the wheel speed and graph line B shows the front brake pressure. Graph line B illustrates the sudden changes in front brake pressure and pressure spikes which are present in the brake line during operation of the ABS. FIG. 1a shows an expanded view, corresponding to area labeled as view "a" in FIG. 1, of the front brake pressure. The pressure spikes shown in FIG. 1a can be as high as 400 to 600 pounds per square inch (psi). A rapid oscillation in a signal or pressure is referred to as "ringing." The expanded view in FIG. 1a also illustrates the ringing which occurs on the brake line. Switching the state of a conventional solenoid valve causes a damped oscillation of the pressure (the ringing) in the brake line. The ringing radiates along the length of the brake line, including under the vehicle, and causes the brake line to physically vibrate. The vibration is perceived by occupants of the vehicle as audible noise. Ideally, it would be desirable to provide a hydraulic or electro-hydraulic braking system in which this audible noised is reduced.